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Mechanisms that control the latitude of jet streams and surface westerlies

Mechanisms that control the latitude of jet streams and surface westerlies. Gang Chen (Princeton University) Isaac Held (GFDL). August 1, 2007 @GFDL. Outline. GFDL climate models: The responses of the jet streams and surface westerlies to ENSO and global warming in austral summer.

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Mechanisms that control the latitude of jet streams and surface westerlies

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  1. Mechanisms that control the latitude of jet streams and surface westerlies Gang Chen (Princeton University) Isaac Held (GFDL) August 1, 2007 @GFDL

  2. Outline • GFDL climate models: The responses of the jet streams and surface westerlies to ENSO and global warming in austral summer. Changes in the eddy momentum flux spectrum. • An idealized dry model: Case I: the jet shift to a decrease in surface friction. Case II: the jet response to prescribed zonal forcing.

  3. Motivations • La Niña is associated with a poleward shift in the SH surface westerlies (e.g. Seager et al. 2003, L’Heureux and Thompson 2006). • There is a positive annular mode trend in the observations in recent decades, which has been attributed in part to stratospheric ozone depletion (e.g. Thompson and Solomon 2002, Gillett and Thompson 2003). • The midlatitude storm tracks are displaced poleward under global warming (e.g. Yin 2005).

  4. Part I: Diagnoses from GFDL climate models

  5. GFDL models GFDL AM2.1(1961-2000, 10 realizations): atmosphere model, with observed SST, sea ice and radiative forcings. CM2.1(2001-2100): coupled model, IPCC A2 scenario. surface-wind-max latitude (SH DJFM) Red: ERA-40 Cyan: model

  6. Methodology • Austral summer (DJFM seasonal mean) • weak Stratosphere-Troposphere Coupling • weak stationary waves • La Niña (regression onto the inverted Cold Tongue Index in AM2.1) • Internal variability (regression onto the internal variability index after removing the ensemble mean in AM2.1) • Global warming (2081-2100 minus 2001-2020 in CM2.1)

  7. Tropopause Climatology La Niña Zonal mean zonal wind--- La Niña C Internal variability

  8. Tropopause Global warming Climatology Zonal mean zonal wind--- global warming C W Internal variability

  9. The eddy momentum flux spectrum In the zonal and seasonal average, the zonal mean zonal momentum balance is s -(vu)/y dp¼ surface stress The eddy momentum flux, vu, can be decomposed as vu = ss vu(k,) dkd = ss vu(k,c) dkdc where k is wavenumber,  is frequency, c=/k is phase speed. (Randel and Held 1991)

  10. Latitude/phase speed spectrum – climatology Eddy momentum flux convergence U/cos at 250 hPa U/cos at surface Red: convergence;Blue: divergence

  11. Latitude/phase speed spectrum – La Niña Equator S. Pole Climatology: shading; La Niña: contours The subtropical divergence shifts poleward due to negative subtropical wind anomalies.

  12. Latitude/phase speed spectrum – global warming Equator S. Pole Climatology: shading; global warming: contours The midlatitude eddies get faster, which can be thought of the result of increased stratospheric winds

  13. wavenumber/phase speed spectrum Eddy momentum flux (45S--55S) La Niña global warming Climatology: shading; Response: contours Red: equatorward flux;Blue: poleward flux

  14. Summary for Part I • In the austral summer, the zonal mean zonal wind displays a poleward shift in the tropospheric jet in response to La Niña and global warming. The differences from the internal variability pattern can be roughly related to thermal forcings. • In the phase speed spectrum, the response to La Niña is consistent with the poleward shift in the critical latitudes due to subtropical wind anomalies, and the global warming trend is associated with an increase in eddy phase speed, which can be related to increased stratospheric winds.

  15. Part II: Studies with an idealized dry model

  16. An idealized dry model • GFDL atmospheric spectral dynamical core • Held-Suarez Physics (Held and Suarez, 1994) • Equinox, No topography • Two case studies: • 1) Troposphere only: T42, Z20 (equally spaced sigma levels) • surface friction (Robinson 1997) • 2) Troposphere + Stratosphere: T42, Z40 (enhanced stratosphere resolution) • prescribed zonal forcing (Song and Robinson 2004; Ring and Plumb 2007)

  17. Case I: surface friction barotropic governor (see James 1987)

  18. U (=0.875): mean drag U (=0.875): eddy drag U (=0.875): total drag eddy drag vs. mean drag The zonal flow and jet latitude are controlled by mean drag.

  19. U (=0.875) -(vu)/y (=0.275) Energy Transient response (30 realizations) Fast adjustment 0-15: abrupt decrease of KE Slow adjustment 15-100: slow poleward jet shift 15-300: slow increase of KM

  20. Slow adjustment Fast adjustment The equilibrated response = initial rapid barotropic acceleration at the jet latitude + the following slow poleward jet shift.

  21. (0.5d), =0.275 (1.5d), =0.275 (1.5d)-(0.5d), =0.275 Eddy momentum flux convergence spectra

  22. A shallow water model • Thermal relaxation to equilibrium thickness (H2eq) • A stochastic stirring that mimics baroclinic instability

  23. Slow eddies Fast eddies Difference Eddy momentum flux convergence spectra

  24. The mechanism for the critical latitude movement 1) Rossby waves prefer to propagate equatorward in the atmosphere. 2) The eddy phase speeds can be modified by the strength of midlatitude jet.

  25. Case II: prescribed zonal forcing Zonal mean zonal wind climatology annular mode

  26. Tropospheric forcing Zonal wind (Full model) Torque Torque (Zonally symmetric model)

  27. Eddy momentum flux convergence spectra Control Torque (=0.85, =30) Torque (=0.85, =50) N. Pole Responses Equator

  28. Stratospheric forcing Torque Torque Zonal wind EP vectors

  29. Jet Torque position and jet movement Poleward shift Equatorward shift

  30. Summary for Part II • Case I (surface friction) shows that the increased eddy phase speed, due to the accelerated westerly jet, can lead to the poleward shift in the critical latitudes of midlatitude eddies. • Case II (zonal forcing) shows that the direction of the jet shift depends on the position of the zonal wind acceleration.

  31. A schematic summary phase speeds subtropical winds

  32. Acknowledgement • Advisor: Isaac Held • General and thesis committee: Isidoro Orlanski, Geoff Vallis, Steve Garner, Lorenzo Polvani (Columbia) • Walter Robinson (UIUC), Paul Kushner (U. Toronto), Gabriel Lau and other AOS Faculty members • Postdocs: Jian Lu, Pablo Zurita-Gotor and others • Fellow students: Dargan Frierson, Edwin Gerber and others

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